I have ford Aod, same as your C-6 but with over drive. The noise you here are coming from the tranny not the rear end. If the kickdown rod or cable are not adjusted properly you will get very hard down shift (clunk) when letting off the gas. Mine is really bad when down shifting from 3rd to 2nd. You may need alittle break-in time before you get it just right. Also you can change your trans. fluid to B&M's
synthetic Trick Shift performance automatic transmission fluid. This stuff is great. I don't know for sure that this will help you problem, but this sounds like the same thing that I encounterd when I got my Cobra on the road.